Edge 5W-50 in an engine specced for 20W-50

Of course a hot 15W-50 drains faster than cold 5W-40, did you read any of the responses in this thread? That isn't what you were asking about.

Here is a graph of the viscosity of a typical 15W-40 plotted against a typical 5W-40 (in Celsius). What do you see?

Again, no oil regardless of the winter rating is ever too thin in cold temperatures. If it isn't too thin when hot it won't be too thin when cold. In fact it will be massively thick.

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Isn't the spread between 5w40 and 15w40 wider at freezing?
5w50 is a fairly new grade its fine in anything that can use a 50 grade. Its only 'thinner' on start up or when cold. It would need to be -50w50 to be anywhere near as thin as it is when hot.
My only concern with 5W-50 is how shear prone I've seen some look (Down in the 30 Grade Range by OCI Time). I think I would be more prone to run a 5W-40 HDEO which tends to go in thinner but come out thicker.
I'm not sure how new (old) your chart is. Back in the 60's/70's when I had my Austin's and MG's any of your 50's were unheard of and unavailable here in the US. 10w-40 was very popular and what was recommended and used. Maybe your chart is from the EU and not one for the US market? Anyways, I used 5w-30 (Mobil1) in my MGB and ran it back and forth to San Diego several times to visit a friend. Ran great and used no oil.

EDIT: I, also, worked at a Jaguar/MG/Datsun dealer back then and we put bulk 10w-40 in everything.
Interesting. I have been driving MG's for over 50 years. My 71 which I kept for a bit over 30 years spec'd 20w50. I have lived all over including what I consider cold, which is outside Denver CO ( where I owned a 66B) and always used a 20w50 without problem. Now, I have know guys that have run all kinds of grades without a problem, but I stuck with what was in my owners manual. It was a 20w50 in my engine and gearbox.
Nothing wrong with M1 5W50, but if I was migrating from a mineral 20W50, my choice would be M1 15W50 as it is more shear stable, less volatile and carries more ZDDP in its add pack.